1. Field of the Invention
The invention pertains to the field of methods and apparatus for materials deposition. More specifically, the materials are deposited as films using a supercritical deposition reactor with recycle of metal organic precursors.
2. Discussion of the Related Art
There are a variety of deposition processes for making thin film or thick film materials. For example, vacuum sputtering processes, spin-on deposition, and chemical vapor deposition processes are used to deposit thin films for use in semiconductors or optics. Chemical vapor deposition (“CVD”) processes are those in which a volatile precursor liquid, typically a metal organic liquid, is vaporized in a carrier gas at reduced pressure and at an elevated temperature below the decomposition temperature of material in the combined flow stream. A chemical reaction may occur to deposit material on a deposition substrate, for example, by thermal activity from a heated wafer or by the activity of plasma in the case of plasma enhanced chemical vapor deposition (“PECVD”).
U.S. Pat. No. 5,789,027 issued to Watkins et al. is hereby incorporated by reference to the same extent as though fully replicated herein. The '027 patent describes chemical fluid deposition (“CFD”) processing where a precursor material is dissolved into a solvent that is maintained in a supercritical state. Solvents for this purpose include carbon dioxide, propane, butane, pentane, dimethyl ether, ethanol, nitrous oxide, water, and hexafluorobenzene. Precursors for use in CFD deposition processes are chosen to produce a desired material on the deposition substrate following reaction with a reaction agent. The reaction reagent may be dissolved in the supercritical solvent, or it may be the supercritical solvent itself. By way of example, materials that may be deposited and/or formed using CFD processes include:                metals, such as Cu, Pt, Pd, and Ti;        elemental semiconductors including Si, Ge, and C;        compound semiconductors including Group III-V semiconductors such as GaAs and InP; Group II-VI semiconductors such as CdS; and Group IV-VI semiconductors such as PbS;        oxides such as SiO2 and TiO2; and        mixed metals and/or mixed metal oxides such as the YBaCuO low temperature superconducting materials.        
CFD processes produce high quality films and offer the advantage of low process temperatures of less than 250° C. or even less than 100° C. On the other hand, the reactor effluent is problematic. The processes are performed in a supercritical reactor vessel that must be pressurized and depressurized to accommodate new wafers for processing. The supercritical solvent contains potentially harmful chemicals that have been partially reacted to deposit materials on the wafers or other deposition substrates. The potentially harmful chemicals frequently contain heavy metals that cannot be destroyed, even by calcination. Upon depressurization, there is a need to vent the supercritical solvent and the chemicals dissolved in the supercritical solvent. Special precautions must be taken to protect the environment from exposure to these chemicals. Processing and disposal of the reactor effluent is complicated by, for example, the tendency of carbon dioxide to expand as a gas. The supercritical solvent with dissolved chemical reagents and reaction byproducts must be renewed for each cycle of deposition processing. This requirement presents a tremendous challenge and expense in accommodating process waste disposal needs.